1. Field of the Invention
The present invention relates to a nanocrystal-metal oxide complex and a method for preparing the complex. More specifically, the present invention relates to a nanocrystal-metal oxide complex with improved luminescent efficiency, superior optical stability and excellent chemical stability which comprises a nanocrystal and metal oxide substituted with two or more surfactants, and a method for preparing the complex.
2. Description of the Related Art
A semiconductor nanocrystal is a crystalline material generally having a particle size (i.e., a particle diameter) of a few nanometers up to about 10 nm, and consists of a cluster of several hundred to several thousand atoms. Such a small-sized semiconductor nanocrystal has a large surface area per unit volume, and therefore most of the constituent atoms of the nanocrystal are present on or near the surface of the nanocrystal. Based on this characteristic structure, a semiconductor nanocrystal exhibits quantum confinement effects and shows electrical, magnetic, optical, chemical and mechanical properties that differ from those inherent to the constituent atoms of the nanocrystal, or from bulk properties of the constituent atoms of the nanocrystal.
Control over the physical size and composition of semiconductor nanocrystals enables the control of the properties of the nanocrystals. Accordingly, semiconductor nanocrystals can be utilized in a variety of applications including: luminescent devices such as light-emitting diodes (“LEDs”), electroluminescence (“EL”) devices, lasers, holographic devices, and sensors; and electronic devices such as solar cells and photodetectors. For various applications, nanocrystals must be incorporated in an appropriate matrix. Accordingly, nanocrystals must exhibit excellent dispersibility and formability in a medium (e.g., a solution) as well as superior luminescent properties.
Nanocrystals are generally prepared by a wet chemistry technique wherein a precursor material is added to a coordinating organic solvent to grow nanocrystals to the desired size. In a wet chemistry technique, as nanocrystals are grown, the organic solvent can coordinate to the surface of the nanocrystals, thus acting as a dispersant for the nanocrystals. Accordingly, the organic solvent allows the semiconductor nanocrystals to grow to nanometer-scale. The wet chemistry technique has an advantage in that nanocrystals having a variety of sizes can be uniformly prepared by appropriately controlling the concentration of precursors used, the kind of organic solvents, and preparation temperature and time, and the like. Also, according to the wet chemistry technique, since nanocrystals have a large surface area per unit volume due to their extremely small size, they are vulnerable to surface defects readily undergo aggregation. The surface defects act as energy traps between energy bandgaps, thereby disadvantageously causing a deterioration in luminescent efficiency.
In an attempt to overcome this, preparation of a nanocrystal-metal oxide complex in which nanocrystals are dispersed in a transparent metal oxide matrix has been proposed as a way to improve the stability of nanocrystals by preventing oxidation or aggregation of nanocrystals, both of which phenomena results from outside stimulus. The surface of nanocrystals prepared by the wet chemistry process is surrounded by an organic surfactant. Based on this, and to allow the nanocrystals to be dispersed in the metal oxide matrix, some methods have been suggested in which the materials coordinated to the surface of the nanocrystals are substituted with a surfactant, which is compatible with metal oxide precursors. For example, methods for preparing nanocrystal-metal oxide complexes by substituting the surface of nanocrystals with an alkyl silane-based surfactant and mixing the nanocrystals with a metal oxide precursor are known, wherein the alkyl silane-based surfactant has a terminal group (such as thiol (—SH), amino (—NH2) or carboxy (—COOH)) capable of binding to the surface of nanocrystals at one end, and a Si(OR)3 terminal group at the other end. International Patent Publication No. WO 2005/049711 discloses a method for preparing nanocrystal-metal oxide complexes by substituting the surface of nanocrystals with a surfactant which has at least one group (e.g., —SH, —NH2 or —COOH) capable of at one end binding to the surface of nanocrystals, and a hydrophilic group (e.g., —OH, —COOH, —NH2, —PO3H2, —SO3H or —CN) at one other end, that is capable of interacting with the solvent.
However, these conventional methods, such as methods for preparing nanocrystal-metal oxide complexes wherein the surface of nanocrystals is substituted with one type of surfactant, have a difficulty in dispersing nanocrystals in a metal oxide matrix while luminescent and electrical properties of the nanocrystals are maintained. In addition, there is a limitation that the nanocrystal-metal oxide complexes prepared by the methods has poor luminescent property and stability.